The recently developed CuInS2/TiO2 3D nanocomposite solar cell employs a three-dimensional, or "bulk", heterojunction to reduce the average minority charge-carrier-transport distance and thus improve device performance compared to a planar configuration. 3D nanocomposite solar-cell performance is strongly influenced by the morphology of the TiO2 nanoparticulate matrix. To explore the effect of TiO2 morphology, a series of three nanocomposite solar-cell devices are studied using 9, 50, and 300 nm TiO2 nanoparticles, respectively. The photovoltaic efficiency increases dramatically with increasing particle size, from 0.2% for the 9 urn sample to 2.8% for the 300 nm sample. Performance improvements are attributed primarily to greatly improved charge transport with increasing particle size. Other contributing factors may include increased photon absorption and improved interfacial characteristics in the larger-particle-size matrix.